h1 {
    font-family: sans-serif;
    text-align: center;
  }
  
body {
    width: 640px;
    margin: 0 auto;
  }

  

.full-img {
    position: relative;
    display: block;
    width: 640px;
    height: 480px;
  }

  .full-img img{
    width: 640px;
    height: 480px;
  }
  
.overlay {
    position: absolute;
    top: 0;
    left: 0;
    width: 640px;
    height: 480px;
    background-color: rgba(0,0,0,0);
    transition-duration: 500ms;
  }
  
button {
    border: 0;
    background: rgba(150,150,150,0.6);
    text-shadow: 1px 1px 1px white;
    border: 1px solid #999;
    position: absolute;
    cursor: pointer;
    top: 2px;
    left: 2px;
  }
  
.thumb-bar img {
    display: block;
    width: 20%;
    height:80px;
    float: left;
    cursor: pointer;
  }